1. Field of the Invention
The present invention relates to a thermal plate-making apparatus. More particularly, the invention relates to a structure of a thermal head which heats to perforate an original sheet for screen printing.
2. Description of the Related Background Art
In conventional thermal plate-making apparatus, a thermal head has a line of heat units at a predetermined pitch in a primary scan direction. Each of the heat units has a heat element between a pair of electrodes opposing to each other in a secondary scan direction perpendicular to the primary scan direction. The secondary scan direction is a shift direction of a thermal screen original sheet composed of a thermoplastic resin film and a porous support. The heat elements are contacted with the thermoplastic resin film to heat to perforate the thermal screen original sheet.
The size of the heat elements would be limited by an amount of shift of the thermal screen original sheet. For example, supposing dimensions of each heat element are a in the primary scan direction and b in the secondary scan direction, one has a ratio of a to b as 1:2, another has longer b than the pitch between two adjacent heat elements, and other has equal a and b. Using such thermal heads with thus formed heat elements, there has been recognized the following problems after perforation of the thermal screen original sheet.
The heat elements are heated while contacting with the thermoplastic resin film to fuse and perforate the film. Such a problem occurs after heating that the fusing thermoplastic resin film is melted too much after the heating of the heat elements due to thermal inertia, causing interconnection of perforations.
Then, when the thus formed thermal screen original sheet is used for screen printing, an excessive amount of ink would be transferred through the interconnected perforations onto a print sheet. The printed sheets with images formed with an excessive amount of ink cause problems when stacked on a sheet tray. One of the problems is back face reprinting, which is reprinting of an image on a printed sheet discharged onto the tray, onto a back face of the next discharged printed sheet. Another problem is rubbing stains caused on a stacked printed sheet which is rubbed by the following printed sheet.
Japanese Unexamined Patent Publication Hei 2-67133 describes a thermal plate-making apparatus preventing the interconnection of perforations by shortening the length b of heat elements in the secondary scan direction as being smaller than the pitch of heat elements to reduce a contact area between the heat elements and thermoplastic resin film, to solve such problems.
In the above arrangement of only one heat element between the opposing electrodes, a smaller diameter of perforation may reduce an amount of ink passed therethrough to prevent the back face reprinting or the rubbing stains to some extent. However, too small perforations result in enlargement of spacing between neighbouring perforations, which in turn causes a dotted image when printed. The dotted image might lack an image density required.
Furthermore, the thermal head as disclosed in the above-mentioned Japanese Unexamined Patent Publication Hei 2-67133 has rises at respective electrodes due to wiring with a height difference. The rises change a condition of contact of a thermal screen original sheet with a protection layer covering the heat elements and the electrodes, so that ununiform perforations would be formed on the original sheet.
In detail, the thermal screen original sheet is composed of the thermoplastic film contacting the surface of the thermal head and the porous support supporting the back of the film. The porous support includes numerous fibers therein.
When the thermoplastic film is brought into contact with the thermal head to form the ink passing perforations, one of the heat elements might be located by chance at a position where there is no fibers in the porous support. In such occasion, there would be caused a spacing between the thermoplastic film and the protection layer of the thermal head, separating them from each other even upon pressing by a platen roller. This causes unstable heat transfer from the thermal head, which results in reduction of perforation size or no formation of perforation.